Two pole contactor

ABSTRACT

A two pole contactor, particularly for a domestic electricity meter, comprising a solenoid with a plunger actuator and a movable contact for each pole mounted on a pivotal blade in a symmetrical opposed configuration. The plunger is connected to the blades by a leaf spring whose ends engage sliders connected to the blades to impart a similar and even movement to each blade.

FIELD OF INVENTION

The present invention relates to a two pole contactor, particularly foruse in domestic electricity meters in which it is desired to have atotal isolation between the utility or electricity supply metering sideand the domestic circuits.

BACKGROUND TO THE INVENTION

The distribution system in North America is such that domestic premisesare fed with a 2-phase (180° phase relationship) utility supply, thelocal transformer centre tap giving an artificial Neutral for normallow-current loads at 115 V, while the voltage across phases is 230 V forpower loads such as air-conditioning, motor drives and heaters. Thelocal transformer primary is usually fed from an overhead fused 25 KVsupply, so that the contactor switch contacts must safely withstand anyreasonable short-circuit fault on the load side of the meter.

Known contactor designs exist for performing such switching functions inassociation with domestic electricity meters used in North America.

In U.S. Pat. No. 4,388,535 the feed connections are provided with setsof fixed pairs of contacts, and related sets of sprung, contactedshorting bars are positioned in proximity to the fixed contact sets,such that when they are actuated the two switch sets make contact,connecting the feed or utility side to the domestic load side.

Actuation is achieved by a moving plunger within a power solenoid coil,and a set of pivoted bellcrank levers operate to push open the sprungshorting bars or to retract to close them, the spring forces providingthe necessary contact closure. A microswitch is used to interrupt thesolenoid coil drive during the OPEN and CLOSE actuation functions,ensuring that the energisation is only momentary, thus preventing thecoil from over-heating and possible burn-out.

In U.S. Pat. No. 4,430,579 the construction is similar to U.S. Pat. No.4,338,535, using sprung contacted shorting bar switch sets to create the2-pole contactor function. But the actuation method adopted is differentin that the solenoid is double-acting, the plunger being naturallyattracted centrally into a power drive coil when energised, this beingthe point of greatest flux concentration. In being attracted centrally,the plunger is dynamically over-driven past its centre to mechanicallylatch at each end of its stroke. The coil power is typically 2,000 W fora reliable double-action mechanical latching function.

This solenoid double-action is used to translate the switching functionvia suitably guided roller-aided push rods, either to CLOSE or OPEN thetwo sprung switch sets, the contact closure force being provided by thecompression springs behind each shorting bar. In order to ensure thatthe contacts do not separate under short-circuit fault conditions, arelatively high force must be applied by each compression spring.

The solenoid plunger is profiled in such a way as to perform both thetranslation and mechanical latching functions simultaneously. A variantof the profiled plunger uses a similarly profiled, hardened steel platesuitably pinned to the plunger, to perform the same mechanicaltranslation and latching functions, respectively. A microswitch is againused to interrupt the solenoid coil drive to prevent the coil fromover-heating.

It is an object of the present invention to provide an improved two-polecontactor.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided atwo-pole contactor comprising a solenoid having a plunger actuator, afixed contact and a moveable contact for each pole, the moveablecontacts being each symmetrically mounted on a pivotal blade, in whichthe plunger is connected to the centre of a leaf spring, whereby in usethe ends thereof impart a similar and even movement to each blade.

According to another aspect of the present invention there is provided acontactor having at least a single pole pair of contacts and a solenoidoperated plunger to actuate the contacts, in which the part of theplunger external to the solenoid is made of non-magnetic material toreduce the influence of the interfering magnetic fields during theexcess current or short-circuit fault conditions.

According to a further aspect of the present invention there is provideda contactor comprising a solenoid with a plunger actuator mounted withina metal frame and biased by a spring to the open condition of thecontactor, the plunger contacting a stop on the frame in the closedcondition, in which the status of the contactor is determined by passinga voltage between the frame and the spring, so that a circuit is madewhen the plunger contacts the stop in said closed condition.

Other features of the invention are defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A contactor in accordance with the invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

FIG. 1 is a plan view of the contactor with the top removed to show theblade assemblies;

FIGS. 2A to 2D are views of a U-frame for the shrouded solenoid, showingrespectively a view from above, a plan view taken on the partial sectionline II—II of FIG. 2A, a side view, and a view from beneath the frame;

FIGS. 3A and 3B are views from one side and beneath respectively of abus-bar assembly incorporating a moving blade; and

FIG. 4 is a plan view showing a status switch in the closed position.

DETAILED DESCRIPTION

Referring first to FIG. 1, the contactor shown is designed to be fittedwithin a domestic electricity meter casing, or into a meter basemoulding at the interface of a house, for isolating the mains utilitypower feed to domestic loads within the house. It may also be integratedinto a proposed automatic meter reading (AMR) pre-payment andcommunication system, with the option of remote disconnection andreconnection of the customer's supply. The contactor comprises a stoutmoulded casing 8 made of an electrically non-conductive material andwhich forms a base into which are mounted two separate balanced andsymmetrical mirror-image switching systems.

In order to avoid unnecessary repetition of references in the drawings,only the left-hand parts of the switch will generally be referred to, itbeing understood that the right-hand parts are essentially similarexcept where specifically stated.

Power is fed to the contactor from an inlet bus-bar 10 which isconnected by a thin spring portion 12 to a bi-furcated moving blade 14having a pair of inlet contacts 16 formed at the ends (see also FIGS. 3Aand 3B). Power is delivered out of the contactor from an outlet bus-bar18 which has fixed double contacts 20 for mating with the inlet contacts16.

Mounted centrally between the ends of the outlet bus-bars 18 is asolenoid actuator 24 comprising a ferrous plunger 26 slidable within asolenoid drive coil 22.

A spigot 28 connected to a yoke 32 engages loosely within an aperture 30in the plunger 26, to which it is connected by a pivot pin 29. At eachend of the yoke 32 the lower face engages with a compression spring 34,while a pair of projections 36 on the upper face engage with a pair ofshaped leaf-springs 38, held at their centre by a pin 39A of a holder 39made of aluminium casting. The end of each spring 38 engages in a slotof a moulded sliding lifter 40 (only one shown) made of an electricallynon-conductive material and of which the upper end engages with the topand bottom sides of the moving blade 14.

It should be pointed out here that the upper spring 38 and the upperlifters 40 are not shown in FIG. 1, and that the layout of the blades 12is not only mirrored, but is symmetrical and balanced about the axis ofthe solenoid actuator 24, thus presenting a consistent deflecting andactuating force via the two pairs of lifters 40 to each set of contactsin turn.

The moving blade 14 is thinned at one end for flexibility and suitablyattached to the bus-bar 10 by soldering, brazing or ultrasonic welding.During manufacture of this assembly it is important not to generateexcess heat, which could seriously distort the shape of, or affect thespring quality of the moving blade. Each assembly is tightly located andcontained in slots and barriers within the moulded casing 8. Suitablebarriers within the casing provide the required safety isolation betweenthe two individual switches which are at mains supply voltage, and thedrive coil 22 which is at low voltage.

The feed bus-bar 10 and moving blade 14 are formed in such a way thatthey lie parallel to each other for a certain distance, with a smalldefined gap between, along their length. A larger gap exists at theflexible attachment of the spring portion 12 where the blade isrelatively weak, to prevent damage when loaded under fault conditions.This blade arrangement is the basis of the so-called “blow-on” layout(as described and claimed in UK Patent Application Serial No. 2295726)[ref. 480.00/B] which is designed to give increased contact force andhence superior switching performance, especially under excessive orshort-circuit current fault conditions.

Under such excessive/short-circuit fault conditions the current in thefeed bus-bar 10 is in the opposite direction to that flowing in therespective adjacent moving blade 14, so that electrodynamic forces aregenerated between them, trying to force them apart. The force isapproximately proportional to the square of the current. Since the feedbus-bar 10 is comparatively rigid, these forces act directly upon themoving blade, thus increasing the forces between the contacts 16, 20over and above the optimal overtravel force which is set when thesolenoid adjustment takes place.

Opposing this increasing blow-on force, and attempting to open thecontacts, is the so-called contact repulsion force, which is related tothe geometry of the current flow through the contacts themselves.

The magnitude of this field-induced repulsion force is alsoapproximately proportional to the square of the current, and is afunction of the ratio of the contacting diameter to the actual contactdiameter. In general the more “bedded” or “conditioned” the contactingsurfaces are, the lower the repulsion forces between them. The effect ofthese two opposing forces is a net increase of the nominal contact forcewith increasing current, thus providing greatly improved and moreefficient switching.

Referring to FIGS. 3A and 3B, the pair of moving blades 14 are shown ina condition in which the bifurcated contacts 16 are open.

Adjacent its contact end the moving blade 14 is formed with a slightlyU-shaped portion 15 so as to freely engage with the sliding lifter 40,one half below and the other half above, for free actuation of theblade. The bottom end of the lifter 40 is engaged with the lower one ofthe two leaf-springs 38 within the holder 39 (only the bottom one beingshown). Both split lifter sets are contained by and run smoothly ingrooves (not shown) within the base and lid mouldings of the contactor.

As the leaf-spring holder 39 is freely pinned to the solenoid actuatorplunger 26, and lies symmetrically between the two lifter/moving bladesystems, this ensures that actuation forces translated from the solenoidplunger to the blades via the two leaf springs 38 are evenly distributedon both sides, thus giving similar, distributed contact forces andreliable switching. Furthermore, as each leaf spring 38 is entrapped bythe central pin 39A, giving three fixing points within the holder 39,one limb on each side being pre-tensioned to exert a slightly greaterpick-up force than the other, the result is that during actuation, onehalf blade contact is slightly advanced with respect to the other,creating an early closure with its mating fixed contact, followedrapidly with closure of its counterpart.

The pre-tensioning is designed in such a way that at the end of thestroke or overtravel, all four contacts 16, 20 receive approximately thesame, consistent nominal contact force. Also, by virtue of the blow-onelectrodynamic forces, a considerably lower nominal contact force isrequired for operation at normal current levels, in this case 200 A rms.Typically, each contact force is in the region of 300 to 400 g (3 to 4Newtons).

This is the basis of a “sacrificial” contact pair on each set; onecontact taking the brunt of the early closure and late opening, with theother contact carrying the load current. In practice, however, bothcontacts should share the load current equally.

The advantages of bifurcated contacts with such a sacrificial contactpair are as follows:

a) Since the total load current is equally shared between the bifurcatedcontact sets, it can be shown that the total heating effect isapproximately halved.

b) Halving of the load current through each pair of “sharing” contactsmore than halves the total resultant contact repulsion force which isattempting to open the contacts.

c) The combined effect of a) and b) above allows a lower leaf springforce to be utilised.

This also makes the blow-on layout less critical, while still giving animproved reliable switching life to the contactor.

The solenoid actuation 24 is latched by rare earth magnets 37 and onlyrequires a short DC pulse for its operating and release functions, thelatched hold force being considerably greater than the total contactforce exerted via the double leaf-springs 38. This surplus hold ensuresthat the contactor function is not susceptible to shock and vibration,or excess current forces.

The actuator thus being magnet latching, and only requiring a shortmomentary DC pulse to perform the operating and release functions, noquiescent power is necessary. This virtually eradicates anyself-heating, as is the case in a non-magnet latching solenoid. Typicalcoil actuation power is only of the order of 20 to 30 W (compared with2000 W for the known contactors cited earlier), with actuation times oftypically 20 ms.

As shown, the solenoid actuator 24 is wound for a single coil, requiringe.g. a positive DC pulse to operate (CLOSE) and a negative DC pulse torelease (OPEN) the contactor switches, and requiring a simplereversing-bridge type of drive circuit. Alternatively, however, thesolenoid may be wound with two coils with a common center tap, requiringDC pulses of the same polarity (say negative going with respect to apositive center-tap common, from separate conducting transistors), so asto achieve the operating (CLOSE) and release (OPEN) contactor functions.

Alternatively in a preferred single coil option, drive is taken directlyfrom the AC supply e.g. via opto-isolated triacs, where it is onlynecessary for a positive half-cycle to operate (CLOSE) and for anegative half-cycle to release (OPEN) the contact function.

In this case, it is advantageous for the triac drive to be triggeredfrom the so-called zero-crossing of the supply, ensuring that thecontacts open and close on a rapidly declining load current (orpreferably at the next zero-crossing), resulting in minimal arcing,enhanced switching and longer contact life.

To assist the release function, the two push-off springs 34 are locatedbetween the leaf spring holder 39 and the contactor casing 8. Thesolenoid axial position is adjustable so that a minimum contact force isachieved, which is then fixed with a pair of screws 54 (see FIG. 4) inholes in the casing, and glued for added retention during the contactorlife. A moulded top cover provided with suitable catches, tightlycontains and integrates the entire assembly within the casing.

Referring now to FIGS. 2A to 2D there is shown a secondary U-frame 42for shrouding the solenoid.

The frame comprises a base 44, a pair of sides 48, from each of whichextends a fixing lug 48, a top side 50 and a lower end 52 having a smallcentral hole 54. The lugs 48 are secured to the moulded base 8 byfasteners, as shown in FIG. 1.

The frame 42 thus consists of a four-sided box structure, which is alsoenclosed at the lower end, and by the aluminium holder 39 beyond itsupper end, thereby excluding large magnetic fields produced by the bladeassemblies during excess or short-circuit fault conditions.

Auxiliary status switch for actuator/contactor function

Some end applications require an auxiliary low-voltage switch, forsignalling to the drive electronics, or indicating remotely, as part ofa pre-payment or Automatic Meter Reading (AMR) system, the status of thecontactor (or at the very least, the status of the solenoid actuator). Asimple version of such a status switch is shown in FIG. 4.

While the contacts 16 and 20 are open, the moving plunger 26 is isolatedin a plastic bobbin from a metal end stop 56 and the solenoid frame 42(at the bottom end) by the stroke distance, typically 2-3 mm. However,the plunger is in continuity with the aluminium leaf-spring holderassembly and both push-off springs 34.

As already mentioned, the functionality of the present contactor reliesupon the successful latching of the magnet solenoid, fundamentallyinvolving a strong, intimate attraction of the metallic plunger 26, thestop 54 and frame 42, when the contacts are closed. This latching holdforce is typically several kilogrammes, and forms an ideal low-voltage,low-current switch.

A wire connection 58 is made to one of the fixing screws 54 for theframe 42, and a similar wire connection 60 is made to the adjacentpush-off spring 34 by means of a tag (not shown) trapped under thespring. The wire connections 58 and 60 are fed to a flag circuit to showthe status of the switch.

When the contactor is in the closed position shown, a continuity loop isformed as shown by the dotted line 62. Thus an electric circuit isformed as follows: from the wire 60 through the spring, along one arm ofthe aluminium yoke 32, through the pivot pin 29 and the plunger 26,across the nickel plated interface with the stop 56, along the side ofthe frame 42, and out from the screw 54 to the wire 58. The wires 58 and60 are fed to a flag circuit to show the status of the contactor, e.g.by a indicator light (not shown).

Immunity to large generated magnetic fields

Some USA and IEC specifications require normal operation of thecontactor following a 6,000 A rms 6 cycle, or a 10,000 A rms ½ cyclefault. During such excessive/short-circuit faults very large magneticfields are generated by the bus-bars 10, the moving blades 14 and loadwiring connections.

The effect of these large magnetic fields is to interfere with orinfluence the standing hold conditions of the magnet latch solenoidwhich in some cases may actually force the solenoid to drop out, openingthe contactor contacts, with catastrophic consequences.

The interfering magnetic fields may enter a magnet latching solenoid inthree ways:

1) by inducing forces via the plunger end face at the leaf springcarrier 39 (which is in close proximity to one of the moving blades),thus directly affecting the nett hold of the solenoid to the point ofdropping out, or

2) by inducing forces directly into the plunger 26 and/or end-stop partswithin the coil area, again affecting the nett hold of the solenoid, or

3) by partially demagnetising conventional existing Ferrite magnets 37momentarily during actuation.

In order to reduce the effect of the large interfering magnetic fieldsat fault conditions the present design provides the following features:

1) The ferrous plunger 26 is shortened so that only themagnetically-active portion is contained within the magnet latchsolenoid, the external actuation portion linking it to the aluminiumleaf-spring holder 39 being non-magnetic eg. insert-moulded plastic oran extension of the holder 39. This considerably reduces the interferinginfluence of the large fault-condition magnetic fields.

2) The rest of the solenoid is shrouded and enclosed by the secondaryU-frame 42, such that further reduction is achieved in the interferinginfluence of the large magnetic fields.

3) The use of rare-earth magnets 37 which not only provide considerablyhigher hold forces, but also makes them inherently difficult todemagnetise because of their greater bulk B.H.max product, which istypically 30 to 35 Mega.Gauss.Oersteds (MGO) compared with 3 to 6 MGOfor the best grades of Ferrite material that are currently used.

The combination of these three improvements is believed to virtuallyeradicate the problem of the magnetic field influence, giving areliable, immune, solenoid performance under the most arduousexcess/short-circuit fault conditions.

What is claimed is:
 1. A two-pole contactor comprising a solenoid havingan actuator plunger, a fixed contact and a movable contact for eachpole, each movable contact being mounted on a free end of a pivotableblade, the two blades being mounted in the contactor in a symmetricalmirror-image arrangement, in which the actuator plunger is connected tothe center of a leaf spring having two ends, each end of the leaf springengaging a respective said blade via a movable member, to thereby impartcorresponding similar movements to the two blades.
 2. A contactor asclaimed in claim 1 in which an end of each blade opposite said free endis connected to a respective inlet bus-bar by a flexible spring portion,each blade and respective bus-bar being disposed in a parallelrelationship, so that in operation electromagnetic forces urge eachmovable contact into closer contact with the respective fixed contact.3. A contactor as claimed in claim 1 in which each blade is divided orbifurcated to provide two movable contacts for each pole.
 4. A contactoras claimed in claim 1 and further comprising a housing formed as amoulding in two halves, so that components of the contactor can beassembled into one of said two halves.
 5. A contactor as claimed inclaim 4 in which each said end of the leaf spring engages with saidmovable member which is connected to a respective blade and which isslidable in a groove of one of said halves of the housing.
 6. Acontactor as claimed in claim 1 in which there are two movable membersfor each pole, one being disposed above and the other below a respectiveblade.
 7. A contactor as claimed in claim 1 in which the solenoid isadjustably mounted by fixed screws for positioning of the plunger.
 8. Acontactor as claimed in claim 1 in which each movable member is made ofan electrically conductive material connected to a respective blade.